1. Field of the Invention
The present invention relates to an electronic appliance that is connected to another electronic appliance by way of a signal line, and more particularly to an electronic appliance that permits another electronic appliance to recognize establishment of connection between the two electronic appliances by applying a predetermined voltage through a pull-up or pull-down resistor to a signal line connecting the two electronic appliances.
2. Description of the Prior Art
An electronic appliance of the type that is connected to another electronic appliance by way of a signal line is often designed to detect establishment of connection between the two electronic appliances by checking the voltage level on the signal line. Typically, the voltage level to be checked for is set, by the addition of a pull-up or pull-down resistor to the signal line, to be equal to the level of the supplied voltage, the level of ground, or a predetermined level somewhere between these two levels.
FIG. 4 illustrates the outline of a conventional circuit design of an electronic appliance of the type that is connected to another electronic appliance by way of a signal line. This figure illustrates how a first electronic appliance 8 recognizes establishment of connection with a second electronic appliance 9 that is achieved by way of a signal line 7. The first electronic appliance 8 has a terminal 81 to which one end of the signal line 7 is connected, a detection circuit 82 that monitors the voltage level at the terminal 81, and a resistor 83 that connects the terminal 81 to a predetermined voltage level. The second electronic appliance 9 has a terminal 91 to which the other end of the signal line 7 is connected, a supplied voltage line 92 that supplies a voltage having a predetermined voltage level Vdd, and a pull-up resistor 93 through which the supplied voltage line 92 is connected to the terminal 91.
Before the signal line 7 is connected between the terminals 81 and 91, the voltage level detected by the detection circuit 82 is equal to the voltage level to which the resistor 83 is connected; for example, it is equal to the ground level. As soon as the signal line 7 is connected between the terminals 81 and 91, the voltage level detected by the detection circuit 82 becomes approximately equal to the predetermined voltage level Vdd, and this causes the electronic appliance 8 to recognize establishment of connection with the electronic appliance 9.
However, according to the circuit design of the second electronic appliance 9 above, the pull-up resistor 93 keeps the terminal 91 and the supplied voltage line 92 connected all the time; accordingly, connecting the signal line 7 between the terminals 81 and 91 is always accompanied by the appearance of the predetermined voltage level Vdd at the terminal 81. That is, establishing physical connection between the two electronic appliances 8 and 9 by way of the signal line 7 is equivalent to establishing logical connection between them that can be detected as the presence of the predetermined voltage level Vdd on the signal line 7. As a result, in cases where the electronic appliance 8 is designed to start a predetermined sequence of operations as soon as it is connected to the electronic appliance 9, or in cases where the electronic appliance 8 is designed to repeat a predetermined sequence of operations as long as it is connected to the electronic appliance 9, there is no other way to start or stop such a sequence of operations in the electronic appliance 8 than to connect or remove the signal line 7 physically.
For example, in a case where the electronic appliance 8 is designed to transmit a signal to the electronic appliance 9 immediately or only a short while after establishment of connection between them, it is only after the electronic appliance 9 has become ready for operation that connection can be established between the two electronic appliances 8 and 9 by way of the signal line 7. A premature attempt to establish connection before the electronic appliance 9 becomes ready for operation results in loss of the signal. Moreover, if an error occurs in the electronic appliance 9 while the electronic appliance 8 is transmitting a signal, the transmission of the signal by the electronic appliance 8 cannot be stopped in any other way than to remove the signal line 7 from the terminal 81 or 91. Since a terminal for connecting such a signal line is typically located at the back of an electronic appliance, connection or removal of the signal line tends to be troublesome, and thus leads to less efficient use of the electronic appliance.